In any radio communications system intersymbol interference (ISI) is caused in the radio path by reflections from objects far away from the receive antenna. The symbols become spread out in time and adjacent symbols interfere with each other. The receiver of the radio communications system must then determine the information that was intended to be sent.
In a GSM system, data is transmitted in bursts, which are placed within timeslots. A training sequence of a known pattern and with good autocorrelation properties is placed in the middle of the data burst. The training sequence is placed in the middle of the burst in order to provide correct channel estimation for the first and the second half of the burst. The position of the received burst in time varies from burst to burst, due to changes in the propagation channel and movement of the mobile station.
In a GSM system a channel equaliser is provided in the receiver. The purpose of the equaliser, placed in the path of the received signal, is to reduce the ISI and multi-path effects as much as possible to maximise the probability of correct decisions. The channel equaliser uses the training sequence in the burst to equalise the multi-path effects. In order to perform the equalization effectively, the receiver must first identify the exact position of the training sequence.
The training sequence is used by the equaliser to create a channel model, which changes all the time but which during one burst can be regarded as constant for a slowly varying channel in time. If two similar interfering signals arrive at the receiver at almost the same time, and if their training sequences are the same, there is no way to distinguish the contribution of each to the received signal. For this reason, different training sequences are allocated to channels using the same frequencies in cells that are close enough so that they do not interfere. When two training sequences differ, and are as little correlated as possible, the receiver can much more readily determine the contribution of each to the received signal.
The receiver knows the training sequence which the transmitter of the radio communications system transmits, and stores such training sequence. By correlating the stored training sequence with the training sequence received from the transmitter, the channel impulse response of the received signal can be measured. The equaliser creates a model of the transmission channel and calculates the most probable receiver sequence.
Conceptually, the equaliser takes the different time-dispersed components, weighs them according to the channel characteristics, and sums them after inserting the appropriate delay between components, so that a replica of the transmitted signal is restored.
The problem in cellular radio becomes more complex due to the dynamic nature of the channel. As the mobile moves through multipath surroundings, the equaliser must continually adapt to the changed channel characteristics. The equaliser knows the transmitted training sequence, and also knows what it has actually received. Thus, the equaliser can make an estimate of the channel transfer function. Thus an adaptive equaliser continuously updates the transfer function estimate, making sure that the decision error does not increase too much during the channel transmission.
In conventional systems, timing estimation is obtained by correlating a data burst with a training sequence stored in the base station. The base station knows the training sequence used by the mobile station. Correlations are performed at various bit positions of the received signal. The bit position that provides the highest correlation value is determined to be the first bit of the training sequence. The received data burst can then be effectively equalized to compensate for the channel.
However, this known technique suffers significantly from the effects of multipath delays in very noisy environments in which there is a low signal-to-noise ratio. Performing the correlation before the equalization leads to errors in timing estimation, and hence bit errors at the output of the equaliser.
It is therefore an object of the present invention to provide an improved technique for estimating the timing position of received data bursts, which operates reliably even in noisy environments.